For a rotating trommel (cylindrical screen), the typical optimum operating speed as a fraction of critical speed is in which range?

Introduction / Context:
Trommels are rotating screens where particle motion and residence time depend on rotational speed relative to the critical speed at which centrifuging would occur.

Concept / Approach:
To promote cascading and adequate presentation to apertures without centrifuging the bed, trommels are typically operated at about one-third to slightly less than one-half of critical speed.

Why Other Options Are Wrong:
Ratios above 1 indicate supercritical rotation causing centrifuging and no screening.Wider ranges include impractical speeds for effective screening.

Final Answer:
0.33–0.45